In single phase induction motors for applications requiring relatively high starting torques, it is quite common to utilize a start capacitor. In such motors, the start capacitor is initially connected to the start or auxiliary winding of the motor to enable a high starting torque to be developed. It is desirable that the start capacitor be disconnected as soon as the motor has started properly and before the high current through the start winding and the stress on the start capacitor can damage the winding and capacitor. It is empirically known that these conditions at which disconnection is desired exist when the motor speed has increased to approximately 80 percent of synchronous speed.
The prior art discloses various electromechanical devices, such as centrifugal switches and relays, for effecting disconnection of the start capacitor. Centrifugal switches have the advantage of being directly responsive to the motor speed. However, because a centrifugal switch is integral with the motor, such a switch is unacceptable in certain applications. For example, such a switch is unacceptable in a hermetically sealed refrigeration compressor apparatus since such a switch could contaminate the refrigerant.
The relays used in the prior art typically are mounted remote from the motor or mounted in a suitable enclosure so as to enable them to be used in conjunction with hermetically sealed apparatus. However, unlike centrifugal switches, relays are not directly responsive to the motor speed. In a typical relay circuit arrangement, the relay coil is connected in parallel with the start winding, and the normally-closed relay contacts and start capacitor are connected in series with each other and in series with the parallel-connected start winding and relay coil. The relay coil is energized to effect opening of its contacts when the voltage across it, which is also the voltage across the start winding, reaches a predetermined pull-in value. It is desired that the predetermined pull-in value occur at the same approximately 80 percent of synchronous speed previously described. Typically, the predetermined pull-in value can be obtained by selecting the proper relay. However, because of variations, such as fluctuations in line voltage, the value of the voltage across the start winding at a specific motor speed will vary. Thus, when the value of the voltage across the start winding is the parameter chosen for effecting relay operation, the motor speed at which the start capacitor is disconnected can vary considerably.
The prior art also discloses totally enclosed electronic starting devices comprising solid state switches, such as triacs, for effecting disconnection of the start capacitor. An advantage of such devices is that, because they are totally enclosed, they can be used in environments such as in hermetically sealed apparatus. Such devices also operate in response to measurement of the value of voltage across the start winding to effect disconnection of the start capacitor at the same approximately 80 percent of synchronous speed previously described. In a typical circuit arrangement utilizing triacs, the triac is in series with the start capacitor and the start winding. The conduction of the triac is controlled by gating circuit means which includes means for compensating for fluctuations in line voltage and corresponding fluctuations in start winding voltage so that the motor speed at which the start capacitor is disconnected remains essentially the same even when line voltage fluctuates. A disadvantage of such triac circuits is that the main terminals of the triac are in series with the start capacitor and the start winding and thus must make, break and carry the relatively large start winding current. Such a large current requirement dictates using relatively expensive triacs.